High-speed steel, cemented carbide and the like have been conventionally employed as compositional materials of substrates for hobs (cutting tools) which are used in a gear cutting. Currently, a coating hob is employed in which a hard coated layer is formed on a surface of a substrate consisting of these materials. Compared with hobs in which a high-speed steel substrate is employed, hobs in which a cemented carbide substrate is employed do not require a cutting oil for cooling during cutting, therefore, offering benefits as being superior with respect to the environment and production efficiency. For this reason, various techniques relating to the cemented carbide materials which are applicable in hob substrates have been made, such as those disclosed in Japanese Examined Patent Application, Second Publication No. S59-52951 and Japanese Unexamined Patent Application, First Publication No. 2001-20029.
According to the techniques disclosed in the above patent documents, it is possible to improve resistance to chipping and resistance to thermal cracking by controlling a content of a βt solid solution in a WC-βt-Co based cemented carbide.
However, in a gear cutting using the hob, there has been a growing demand to increase a hobbing speed, for a purpose of improving production efficiency. It is found that when using hobs in which the cemented carbide disclosed in the aforementioned patent documents are employed under high-speed cutting conditions of 350 m/min or more, chipping readily occurs due to microscopic vibrational movements accompanying high-speed rotation; as a result, a useable lifecycle of these hobs is reached within a comparatively short period of time.